Surprised no one here has mentioned Twist. https://twistapp.com/ We're currently evaluating it in lieu of Slack on our team of ~10 people because of the vast array of issues Slack has. I don't know if it's something that could be considered for larger organizations, but the way it puts together channels and threads is very intuitive, and cuts out a lot of noise for individuals.
In terms of Slack's misgivings, I don't think I've used a more incredibly bloated, buggy, slow electron app. Multiple team logins with no centralized account is _insane_ to mange across teams, this single point frustrates me and most people I talk to a lot. Not to mention the more teams you have, the more your electron app slows to an absolute crawl. The notifications leave a lot to be desired in terms of granularity per channel. They've done a really terrible job on "threads", the UI around it is messy and confusing, and the concept doesn't inline well. Those are just the most apparent daily gripes.